In a spread-spectrum communication system, the transmitted signal bandwidth is much greater than the bandwidth or rate of information being sent. Also, some function other than the information being sent is employed to provide the resulting modulated radio-frequency (RF) bandwidth.
The desired signal is recovered by remapping the expanded-bandwidth signal into the original information bandwidth. In direct sequence CDMA (DS-CDMA) systems, removal or demodulation of the spectrum-spreading modulation is accomplished by multiplication with a local reference identical in structure and synchronized in time with the received signal. Correlation is a method of time-domain analysis that is particularly useful for detecting signals buried in noise, establishing coherence between random signals, and determining the sources of signals and their transmission times. In DS-CDMA systems, the prime purpose of a correlator is to match the local reference signal to a desired incoming signal and thereby reproduce the embedded information-bearing carrier as an output.
Frequency reuse is the process of using the same frequency in multiple separate geographic regions for a plurality of distinct communication links. Frequencies can be reused provided that the regions are attenuated or isolated from each other by a minimum value for signal rejection by user receivers in each region. U.S. Pat. No. 4,901,307 describes the process of creating marginal isolation, which provides an increase in frequency reuse in DS-CDMA systems. In DS-CDMA, even small reductions in the overall power level of the system allow for increased system capacity. One particularly effective method for creating isolation and improving frequency reuse is spatial division multiple access (SDMA). SDMA applications to multiple access communication systems including adaptive array processing are discussed in U.S. Pat. No. 5,642,353, U.S. Pat. No. 5,592,490, U.S. Pat. No. 5,515,378, and U.S. Pat. No. 5,471,647. In addition to frequency reuse, antenna arrays also increase processing gain and improve interference rejection.
The advantage of using adaptive antenna arrays for DS-CDMA communications is that adaptive antenna arrays could provide significant improvements in range extension, interference reduction, and capacity increase. To identify a particular user, a DS-CDMA system demodulates Walsh codes after converting the received signal from RF to digital. Therefore, an adaptive antenna array requires information about the user codes from CDMA radio, or it needs to demodulate many different incoming RF signals to track mobile users. These methods are complex processes and are more difficult to implement than tracking users in non-CDMA systems. Major changes in CDMA radio architecture are required to implement adaptive array processing. These changes may be the major obstacle for adaptive array deployment in the near future.
Phased-array antenna systems employ a plurality of individual antennas or subarrays of antennas that are separately excited to cumulatively produce a highly directional electromagnetic wave. The radiated energy from each antenna element or subarray has a different phase so that an equiphase beam front (the cumulative wave front of electromagnetic energy radiated from all of the antenna elements in the array) travels in a selected direction. The difference in phase or timing between the antenna's activating signals determines the direction in which the cumulative wave front from all of the individual antenna elements is transmitted. Similarly, phase analysis of received electromagnetic energy detected by the individual array elements enables determination of the direction from which received signals arrive.
Beamforming, which is the adjustment of the relative phase of the actuating signals for the individual antennas, can be accomplished by electronically shifting the phases of the actuating signals. Beamforming can also be performed by introducing a time delay in the different actuating signals to sequentially excite the antenna elements that generate the desired transmission direction. However, phase-based electronically controlled phased-array systems are relatively large, heavy, complex, and expensive. These electronic systems require a large number of microwave components (such as phase shifters, power splitters, and waveguides) to form the antenna control system. This arrangement results in a system that is relatively lossy, electromagnetically sensitive, hardware-intensive, and has a narrow tunable bandwidth.